As experienced developers, we are already familiar with good testing practices. Even so, it never hurts to be reminded about some of the more critical habits to maintain.
Any modification greater than trivial should be tested in one way or another by at least two people. The people that are assigned should not be a part of the team that created the design or coded the modification. It would be best if one of the testers is an experienced user, because users seem to have a knack (for obvious reasons) of understanding how the modification operates compared to how the rest of the system acts in the course of day-to-day work. This helps us obtain meaningful feedback on the user interface before going into production.
One of the testing goals is to supply unexpected data and make sure that the modification can deal with it properly. Unfortunately, those who were involved in creating the design will have a very difficult time being creative in supplying the unexpected. Users...